Chapter2
Thermodynamics of Separation
Operations
Purpose and Requirements:
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Know the importance and mechanism of separation
Learn to select feasible separation process
Key and Difficult Points:
Key Points
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Phase Equilibria: Fugacities and activity Coefficients
Graphical Correlations of Thermodynamic Properties
Calculation of K-value
Difficult Points
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Nonidea Thermodynamic Property Modes
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Activity Coefficient Models for the Liquid Phase
Outline
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2.1 ENERGY, ENTROPY, AND AVAILABILITY -BALANCES
2.2 PHASE EQUILIBRIA
2.3 IDEAL GAS, IDEAL LIQUID SOLUTION MODEL
2.4 GRAPHICAL CORRELATIONS OF THERMODYNAMIC PROPERTIES
2.5 NONIDEAL THERMODYNAMIC PROPERTY MODELS
2.6 Activity Coefficient Models for the Liquid Phase
2.1ENERGY, ENTROPY, AND
AVAILABILITY -BALANCES
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Gas Mixture (Solutes or Absorbate)
Liquid (Solvent or Absorbent)
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Separate Gas Mixtures
Remove Impurities, Contaminants, Pollutants, or
Catalyst Poisons from a Gas(H2S/Natural Gas)
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Recover Valuable Chemicals
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2.2 PHASE EQUILIBRIA
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A = L/KV
Component
Water
Acetone
Oxygen
Nitrogen
Argon
A = L/KV
1.7
1.38
0.00006
0.00003
0.00008
K-value
0.031
2.0
45,000
90,000
35,000
•Larger the value of A，Fewer the number of stages required
•1.25 to 2.0 ，1.4 being a frequently recommended value
2.3 IDEAL GAS, IDEAL LIQUID
SOLUTION MODEL
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Stripping
Distillation
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Stripping Factor
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(S解吸因子)
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S = 1/ A= KV/L
High temperature
Low pressure is desirable
Optimum stripping factor ：1.4.
6.1 EQUIPMENT
trayed tower
packed column
bubble column
spray tower
centrifugal contactor
Figure 6.2 Industrial Equipment for Absorption and Stripping
Trayed Tower
(Plate Clolumns板式塔)
Figure 6.3 Details of a contacting tray in a trayed tower
(a) perforation
(b) valve cap (c) bubble cap
(d) Tray with valve caps
Figure 6.4 Three types of tray openings for
passage of vapor up into liquid
Froth
Liquid carries no vapor bubbles
to the tray below
Vapor carries no liquid droplets
to the tray above
No weeping of liquid through the
openings of the tray
(a) Spray(b) Froth(c) Emulsion(d) Bubble(e)Cellular Foam
Equilibrium between the exiting
vapor and liquid phases
is approached on each tray.
Figure 6.5 Possible vapor-liquid flow regimes for a contacting tray
Packed Columns
Figure 6.6 Details of internals
used in a packed column
Packing Materails
•More surface area for mass transfer
•Higher flow capacity
•Lower pressure drop
(a)
(b) Random
Structured
Packing
Packing
Materials
Materials
•Expensive
•Far less pressure drop
•Higher efficiency and capacity
Figure 6.7 Typical materials used in a packed column
SUMMARY
1. Separation processes are often energy-intensive. Energy
requirements are determined by applying the first law of
thermodynamics. Estimates of minimum energy needs can be
made by applying the second law of thermodynamics with an
entropy balance or an availability balance.
2. Phase equilibrium is expressed in terms of vapor-liquid and
liquid-liquid AT-values, which are formulated in terms of
fugacity and activity coefficients.
3. For separation systems involving an ideal gas mixture and an
ideal liquid solution, all necessary thermodynamic properties
can be estimated from just the ideal gas law, a vapor heat
capacity equation, a vapor pressure equation, and an
equation for the liquid density as a function of temperature.
4. Graphical correlations of pure-component thermodynamic
properties are widely available and useful for making rapid,
manual calculations at near-ambient pressure for an ideal
solution.
5. For nonideal vapor and liquid mixtures containing nonpolar
components, certain P-u-7'equation-of-state models such as SR-K, P-R, and L-K-P can be used to estimate density, enthalpy,
entropy, fugacity coefficients, and k-values.
6. For nonideal liquid solutions containing nonpolar and/or polar
components, certain free-energy models such as Margules, van
Laar, Wilson, NRTL, UNIQUAC, and UNIFAC can be used to
estimate activity coefficients, volume and enthalpy of mixing,
excess entropy of mixing, and k-values.
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